1. Field of the Invention
This invention relates to a recipient designed in such a way as to contain a cooling or heating energy storage medium with high latent heat of fusion, and used in energy storage installations.
2. History of the Related Art
Installations requiring the use of variable quantities of heat or refrigeration production over a given period are already known, these installations being designed in such a way as to considerably reduce the capacity of the machines used to produce the heating and/or refrigeration. An example of such a system is described, in particular, in French Pat. No. 79 28 315, filed by Mr. Jean Patry on the 16th of Nov. 1979, for Improvements to Refrigeration Systems (Perfectionnements apportes aux installations "frigorifiques").
In such installations, storage systems are used for the heating and cooling which are produced, then redistributed according to requirements, the storage capacity having been chosen so that the installation may instantaneously supply the maximum capacity demanded by the loads of the installation.
Numerous recipients for installations designed for storing heating or cooling energy are known as well, these recipients containing a phase changing material.
After careful studying of earlier documents covering the state of the above mentioned technique, one can deduce that three types of recipients for phase changing materials exist which are:
(a) a recipient preferably completely filled with the storage medium and having a flexible structure to absorb the variations in volume following a change in volume of the storage medium during its phase changing.
Such an example is described in U.S. Pat. No. 2,525,261.
(b) a recipient preferably completely filled with the storage medium and having an elastic structure to absorb the variations in volume following a change in volume of the storage medium during a phase transformation of the latter.
An example of such a recipient is shown in FR Pat. No. 1,104,404.
(c) a recipient partly filled with the storage medium, and which generally has a rigid structure; the free gas or air space serves, through compression, to absorb the increase in volume due to the change in volume of the storage medium during its phase transformation.
One can find a description of this recipient in U.S. Pat. No. 4,205,656.
In all instances, the aim is to encapsulate a substance which changes phases (water, saline hydrate, paraffin, etc.) within a shell for the storage of heat or cooling energy through the use of the latent heat of fusion.
Due to its symmetrical qualities, a spherical shape for the shell containing the substance which changes phases is by far the most often used.
In addition, bulk loading of these spherical capsules into a tank is simple to achieve, gives a good filling ration for the tank and proper circulation of the coolant effecting the heat transfer.
The thermal qualities of this type of storage are measured by the exchange capacity between the substance which changes phases and the coolant surrounding the capsule.
For a given substance which changes phases and a given temperature difference between the substance which changes phases and the coolant, the exchange capacity is:
proportional to the thermal conductivity of the materials used .lambda.. PA0 inversely proportional to the thickness of the shells or the materials to be crossed e. PA0 proportional to the heat exchanging surface between the substance which changes phases and the coolant S. EQU P=f(.lambda..multidot.1/e.multidot.S) PA0 The technical and economic goal consists of finding the best compromise (.lambda., 1/e, S) at the best cost.
Certain known solutions concerned the thermal conductivity of the materials used in the manufacture of the sphere but, if steel has good thermal conductivity, its cost is high; when a plastic substance is used, thermal conductivity is low and so is the cost price; and, when a compromise consisting of a plastic substance with incorporated metallic particles is used, average thermal conductivity is obtained at a higher price.
Other solutions have consisted in making shells with hollows imprinted on all or part of the sphere to reduce the thickness e to be crossed in order to increase the crystallization speed of the phase substance which changes phases.
There again, difficulties were encountered because the base of the hollow imprints stretch when deformed and become thinner and cause splitting zones and deterioration of the shells.
There are also spheres with an air pocket to absorb expansion of the substance which changes phases without bursting the shell but there again the efficiency is lower due to the air pocket and the increased thickness of the shell which must be rigid.
None of the previous solutions have therefore given satisfaction.